Low temperature distributed feedback laser with loss grating and method

ABSTRACT

A low threshold distributed feedback (DFB) laser is constructed for improved performance at subzero temperatures. A loss grating is employed to enhance the probability that lasing occurs near the short wavelength side of the stopband and to counteract the effect of negative gain tilt that occurs when DFB lasers are positively detuned. A method of making DFB lasers from wafers with improved yield for low temperature side mode suppression ratio (SMSR) is also disclosed.

This application is a divisional of application Ser. No. 09/437,424,filed on Nov. 15, 1999 now U.S. Pat. No. 6,477,194, which is herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates to semiconductor lasers and methods ofmaking and using such devices, and more particularly, to distributedfeedback (DFB) lasers that operate satisfactorily at low temperatures.The invention also relates to a method of making DFB lasers fromsemiconductor wafers and the like.

BACKGROUND OF THE INVENTION

Distributed feedback (DFB) lasers may be used, for example, as sourcesof signal radiation for optical fiber communications systems, as opticalpumps, and as devices for generating coherent optical pulses. In aconventional DFB device, feedback is provided in the longitudinaldirection (the emission direction) by an index grating (a periodic arrayof materials having different optical indices). In another type of DFBlaser, a loss grating is used to provide a periodic variation in loss inthe longitudinal direction.

There is a need in the art for semiconductor DFB lasers that provideimproved performance under conditions, such as low temperature, wherethey would be positively detuned and suffer degradation in side modesuppression ratio (SMSR, discussed in more detail below).

FIG. 1 shows the relationship between output light intensity I andwavelength λ for a typical DFB laser, where: λ_(m) corresponds to theside mode on the short wavelength side of the stopband; λ_(o)corresponds to the selected lasing mode; and λ_(p) corresponds to theside mode on the long wavelength side of the stopband. With reference toFIG. 1, the lasing symmetry L_(sym) for a particular device may bedefined as follows:

L _(sym)=(λ_(p)−λ_(o))/(λ_(p)−λ_(m)).  (1)

According to equation (1), if the lasing symmetry L_(sym) is smallerthan 0.5, then λ_(o) is closer to λ_(p), and if L_(sym) is greater than0.5, then λ_(o) is closer to λ_(m).

The loss function for a DFB laser may be expressed in terms of thecoupling coefficient κ, as follows:

κ=κ¹ +jn, where  (2)

κ¹represents the real part of the coupling coefficient κ, and jnrepresents the imaginary part of the coefficient κ. In a pure indexgrating DFB laser, jn=0, and there is an equal probability of lasing onthe long or short wavelength sides of the stopband, as discussed in moredetail below.

The term “side mode suppression ratio”(or sub-mode suppression ratio)refers to the ratio of main longitudinal mode power to side longitudinalmode power. Some high capacity fiber optic communications systemsrequire a light source that generates a single longitudinal laser mode.A communications system may require a side mode suppression ratioexceeding 30 dB, for example. For a DFB laser to operate in a singlelongitudinal mode, the side longitudinal modes should be suppressed torelatively insignificant power levels.

The side mode suppression ratio (SMSR) changes when the laser is tunedto different operating wavelengths. A DFB laser may exhibit acceptableside mode suppression at certain wavelengths, but unacceptable side modesuppression when tuned to other wavelengths. A small tuning change maycause a 10 dB to 20 dB decrease in the SMSR. This is because therelative net threshold gain required for each mode varies as the laseris tuned. When the main longitudinal mode is centered within thereflection characteristics of the laser, side mode suppression isusually optimized. As the device is tuned away from the optimumposition, one of the side longitudinal modes is moved closer to thecenter of the reflection characteristics.

SUMMARY OF THE INVENTION

A distributed feedback (DFB) laser having a lasing symmetry L_(sym)>0.5provides improved performance (high SMSR) under conditions where itwould be positively detuned. An example of a condition where a DFB laserwould be positively detuned is at low temperatures (for example, −15° C.to −40° C.). Thus, the production yield of DFB devices that perform wellat low temperature can be increased by forming them in such a way as toincrease the likelihood that the devices have lasing symmetries greaterthan 0.5.

A DFB laser is “positively detuned” when its lasing wavelength (selectedmode) is on the long wavelength side of the peak of the material gainspectrum. If the lasing mode is on the long wavelength side, theinherent gain margin is reduced by the relatively large negative gaintilt (dg/dλ) on the long wavelength side. By adding a small amount ofloss to the laser grating (that is, by making jn>0), the likelihood thatthe device will lase on the long-wavelength side of the stop band isreduced. In other words, a DFB laser with a loss grating is more likelyto have a lasing symmetry L_(sym)>0.5. If more devices from a wafer areproduced with L_(sym)>0.5, then the yield to the low temperatureperformance requirement is increased.

The present invention relates to a method of making low thresholdsemiconductor DFB lasers for low temperature operation. The methodincludes the steps of: (1) providing a multi-layer semiconductorstructure, such as a wafer, having an active layer, a loss grating, anda spacer layer; and (2) cleaving the multi-layer structure such thatopposed facets intersect the loss grating. The cleaving process is suchthat the precise location at which a particular facet intersects theloss grating cannot be controlled in advance. As discussed in moredetail below, the performance characteristics of the laser depend inpart on the phase relationships between the randomly determinedlocations of the facets and the periodic structure of the loss grating.

According to a preferred embodiment of the invention, the facets may becovered by highly reflective and anti-reflective coatings, and otherstructures such as claddings, substrates, electrodes, etc. may also beprovided.

According to another aspect of the invention, the loss grating may begrown on a semiconductor substrate by a metal organic chemical vapordeposition (MOCVD) process. In a preferred embodiment of the invention,the loss grating has a high As content and therefore is well suited forreliable growth according to an MOCVD process. In another embodiment ofthe invention, the loss grating may be formed by molecular beam epitaxy.

According to another aspect of the invention, lasers are constructed toprovide high side mode suppression under conditions where they would bepositively detuned. An example of such conditions is where the operatingtemperature is less than about minus fifteen degrees Celsius (−15° C.).

The present invention also relates to a DFB laser for low temperaturesingle mode operation, including: an active layer for producing opticalgain; a loss grating for shifting the emission spectrum to the shortwavelength side of the stopband; and a spacer layer located between theactive layer and the loss grating. According to a preferred embodimentof the invention, the device has a lasing symmetry L_(sym)>0.5.Consequently, the device does not lase on the long wavelength side ofthe stopband under low temperature conditions. The low operatingtemperature may be, for example in the range of from −15° to −40° C.

According to another aspect of the invention, a DFB laser is constructedto have a high SMSR, to operate at a low threshold, and to exhibitminimal mode hopping, all at low temperatures.

According to another aspect of the invention, a device is provided thatoperates in a single longitudinal mode, with a large gain thresholddifference, and that has facet-reflectivity-independent parameters, andlow sensitivity to feedback.

In a preferred embodiment of the invention, the problems of the priorart are overcome by using a loss grating to shift the distribution oflasing modes, generated by random facet phases, to the short wavelengthside of the stopband. The loss grating counteracts the effect of thenegative gain tilt that occurs in DFB lasers due to positive detuning atsubzero temperatures. According to the present invention,temperature-induced variations of the operational characteristics of aDFB laser are minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the mode intensity distribution for a typicaldistributed feedback laser.

FIG. 2 is a schematic side view of a DFB laser constructed in accordancewith a preferred embodiment of the present invention.

FIG. 3 is a histogram showing the distribution of lasing symmetries forDFB lasers with index gratings.

FIGS. 4 and 5 are histograms showing the distribution of lasingsymmetries for DFB lasers with loss gratings, where jn=0.1 and 0.2,respectively.

FIG. 6 is a flowchart for a method of making semiconductor lasersaccording to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings, where like reference numerals designatelike elements, there is shown in FIG. 2 a distributed feedback (DFB)laser 10 constructed in accordance with a preferred embodiment of thepresent invention. The device 10 includes a semiconductor substrate 12,a loss grating 14, a spacer layer 16, an active region 18 and a claddingstructure 20. The cladding structure 20 may be formed of plural layersin a manner known in the art.

In addition, the device 10 has front and back facets 22, 24. The facets22, 24 may be formed by cleaving the device 10 from a wafer (not shown).An anti-reflective (AR) layer 26 is coated on the front facet 22. Ahighly reflective (HR) layer 28 is coated on the back facet 24. Thethicknesses of the two coatings 26, 28 correspond to the wavelength ofthe emitted light 30.

In operation, an electrical current is caused to flow through the activeregion 18 in a known manner. The current causes the active region 18 togenerate light (or gain), such that a signal 30 is emitted through theAR layer 26. The signal 30 may have a wavelength in the range of from1.3 μm to 1.6 μm for use in an optical communications system, forexample. As discussed in more detail below, the illustrated laser 10performs satisfactorily, with high side mode suppression, even at lowtemperatures, for example, in the range of from −15° C. to −40° C.

The substrate 12 may be formed of doped InP or another suitablesemiconductor material. The loss grating 14 may be formed of InGaAsP,with a high As mole fraction, and is preferably lattice matched to thesubstrate 12 with a band gap that is 50 nm to 100 nm greater than thetargeted lasing wavelength. The loss grating 14 has an alternating,periodic structure in the longitudinal direction of the device 10 (thelongitudinal direction is the direction of light emission). The gratingperiod Λ is preferably in the range of from about 2,000 Å to about 2,500Å. The thickness t of the loss grating 14 may be in the range of fromabout 200 Å to about 500 Å, for example.

The spacer layer 16 is located between the loss grating 14 and a lowerportion of the active region 18. The spacer layer 16 may be formed ofInP. In the illustrated embodiment, the thickness 32 of the spacer layer16 may be in the range of from about 3,000 Å to about 6,000 Å. Thepresent invention should not be limited, however, to the details of thepreferred embodiments described herein.

The active region 18 may be formed of bulk InGaAsP and/or multiplequantum wells. There may be for example, five to nine quantum wells witha 1% compressive strain. Each quantum well layer may have a width ofabout 70 Å.

Since the device 10 is cleaved from a wafer, the cleaved facets 22, 24intersect the periodic grating 14 at random, arbitrary locations. When aplurality of devices 10 are cleaved from a single wafer (not shown), theperformance of each device 10 depends on the randomly determinedlocations of the facets 22, 24 relative to the grating 14. The lossgrating 14 is formed across the entire wafer before the cleavingoperation. Consequently, the lasers 10 have facets 22, 24 intersectingthe periodic loss grating 14 at different phases Ø. Some devices 10 willperform differently than others, depending on the phases Ø of the lossgrating 14 at the points of intersection with the facets 22, 24.

The relationship between the facet reflectivity R on either end of thedevice 10 and the respective facet phase Ø is as follows:

R=e ^(iØ)  (3)

By employing equation (3), the expected lasing symmetry L_(sym) for acleaved laser 10 can be calculated for each possible combination offacet phases Ø. FIGS. 3-5 show histograms of L_(sym) values fordistributions calculated by varying the phases Ø at the ends ofdifferent devices in steps of 10° , from 0° to 350°, for a total of 1296facet phase combinations for each distribution. For each histogram, thelasing symmetry distribution is characterized as a function of μ and σ,where μ represents the calculated numerical median of the distributionand σ represents the standard deviation, on the assumption that it wouldbe normal.

FIG. 3 shows the distribution of lasing symmetries for DFB lasers withpure index gratings (κ is real). For κL=1.5, the calculated median isμ₁=0.4998, at a calculated standard deviation σ₁=0.1573. FIGS. 4 and 5,in contrast, show the distribution of lasing symmetries for DFB lasers10 with loss gratings 14, where jn=0.1 and 0.2, respectively. In theFIGS. 4 and 5 distributions, the complex part jn of the couplingcoefficient K is nonzero because the bandgap of the grating 14 isgreater than the wavelength of the emitted radiation, and absorptionoccurs.

As shown in FIG. 4, the lasing symmetry distribution is shifted, and thecalculated median is higher than the median for a pure index grating DFBlaser because λ_(o) is closer to λ_(m.) The calculated median for FIG.4, where jn=0.1, is μ₂=0.5859, at a standard deviation σ₂₌0.1524.

With respect to FIG. 5, where the loss is increased relative to FIG. 4,the combination of facet phases yielding a lasing mode on the longwavelength side of the stopband is smaller than that calculated for FIG.4. For FIG. 5, the calculated median is μ₃=0.6435, considerably higherthan the FIG. 4 value, at a standard deviation σ₃=0.1322.

Thus, the loss grating 14 may be used to increase the probability that alaser 10 cleaved from a wafer (not illustrated) operates on the shortwavelength side of the stopband. As demonstrated in FIGS. 4 and 5, thepercentage of devices that lase on the long-wavelength side is smaller,and consequently the yield for high SMSR at low temperature increases.Because of the more advantageous distribution for low temperatureperformance, an increased number of low temperature devices can becleaved from a single wafer.

Referring now to FIG. 6, a wafer (not illustrated) may be constructed100 with layers corresponding to the substrate 12, loss grating 14,spacer 16, active layer 18, and cladding structure 20. The loss gratinglayer advantageously may be formed by an MOCVD process. Each layer ofthe wafer (including the loss grating layer) may extend with a constantthickness across essentially the full extent of the wafer. The wafer maythen be cleaved 102 into a large number of devices. The cleaving process102 forms the facets 22, 24 at different phases Ø of the loss grating14. The facets 22, 24 are then coated 104 with the desired coatings 26,28, and the finished lasers 10 are then tested 106 for performancecharacteristics. The lasers 10 that will perform well at lowtemperatures may be selected or identified (by, for example, analyzingthe DFB mode spectroscopy) for appropriate low temperature applications.A spectrum analyzer may thus be used to select only the shortwavelengthdevices cleaved from the wafer.

The above description illustrates preferred embodiments which achievethe objects, features and advantages of the present invention. It is notintended that the present invention be limited to the illustratedembodiments. Any modification of the present invention that comes withinthe spirit and scope of the following claims should be considered partof the present invention.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method of making semiconductor lasers for lowtemperature operation, said method comprising the steps of: providing amulti-layer semiconductor structure having an active layer, a lossgrating of a material having a band gap greater than a target wavelengthof the semiconductor lasers, and a spacer layer, said spacer layer beinglocated between said active layer and said loss grating; and cleavingsaid multi-layer semiconductor structure such that opposed facetsintersect said loss grating.
 2. The method of claim 1, furthercomprising the step of locating highly reflective and anti-reflectivecoatings on said facets.
 3. The method of claim 2, further comprisingthe step of forming said loss grating on a semiconductor substrate. 4.The method of claim 3, wherein said loss grating is formed by metalorganic chemical vapor deposition or molecular beam epitaxy.
 5. Themethod of claim 3, further comprising the step of forming asemiconductor cladding structure on said active layer.
 6. The method ofclaim 3, wherein said semiconductor lasers are constructed to providehigh side mode suppression in a positively detuned state.
 7. A method ofproducing a plurality of distributed feedback semiconductor lasers, saidmethod comprising the steps of: providing a multi-layer semiconductorwafer having an active layer, a loss grating, and a spacer layer, saidspacer layer being located between said active layer and said lossgrating; and cleaving said multi-layer semiconductor structure such thatopposed facets intersect said loss grating, said loss grating shiftingan emission spectrum of the lasers to a short wavelength side of astopband.
 8. The method of claim 7, further comprising the step oflocating highly reflective and anti-reflective coatings on said facets.9. The method of claim 8, wherein said loss grating has a grating periodin the direction of light emission in the range of from about twothousand angstroms to about two thousand five hundred angstroms.
 10. Themethod of claim 9, further comprising the step of forming said lossgrating on a semiconductor substrate by metal organic chemical vapordeposition.
 11. The method of claim 10, further comprising the step offorming a semiconductor cladding structure on said active layer.
 12. Themethod of claim 10, wherein said spacer layer has a thickness in therange of from about three thousand angstroms to about six thousandangstroms.
 13. The method of claim 12, wherein said lasers provide highside mode suppression when operated at less than −15° C.
 14. The methodof claim 7, further comprising the step of using a spectrum analyzer toidentify said lasers.
 15. A method of making semiconductor lasers forlow temperature operation, said method comprising the steps of:providing a multi-layer semiconductor structure having an active layer,a loss grating, and a spacer layer, said spacer layer being locatedbetween said active layer and said loss grating; and cleaving saidmulti-layer semiconductor structure such that opposed facets intersectsaid loss grating, wherein said semiconductor lasers are constructed toprovide high side mode suppression in a positively detuned state.
 16. Amethod of producing a plurality of distributed feedback semiconductorlasers, said method comprising the steps of: providing a multi-layersemiconductor wafer having an active layer, a loss grating, and a spacerlayer, said spacer layer being located between said active layer andsaid loss grating; and cleaving said multi-layer semiconductor structuresuch that opposed facets intersect said loss grating, wherein saiddistributed feedback semiconductor lasers provide high side modesuppression when operated at less than −15° C.
 17. A method of producinga plurality of distributed feedback semiconductor lasers, said methodcomprising the steps of: providing a multi-layer semiconductor waferhaving an active layer, a loss grating, and a spacer layer, said spacerlayer being located between said active layer and said loss grating; andcleaving said multi-layer semiconductor structure such that opposedfacets intersect said loss grating, wherein respective phaserelationships between said opposed facets and said loss grating are suchthat said lasers have a lasing symmetry L_(sym) that is greater than0.5.
 18. The method of claim 17, further comprising the step of locatinghighly reflective and anti-reflective coatings on said facets.
 19. Themethod of claim 18, wherein said loss grating has a grating period inthe direction of light emission in the range of from about two thousandangstroms to about two thousand five hundred angstroms.
 20. The methodof claim 19, further comprising the step of forming said loss grating ona semiconductor substrate by metal organic chemical vapor deposition.21. The method of claim 20, further comprising the step of forming asemiconductor cladding structure on said active layer.
 22. The method ofclaim 20, wherein said spacer layer has a thickness in the range of fromabout three thousand angstroms to about six thousand angstroms.